Phase-locked loop control systems (PLLs) are used in a wide variety of applications requiring an output signal with a phase locked to the phase of an input signal. PLLs can be used in clocking systems, for instance. PLLs include a variable frequency oscillator (typically a voltage-controlled oscillator—VCO) and a control loop. The control loop compares the phase of the output signal received from the VCO to the phase of the input signal. A control signal (typically a voltage) is generated based on the comparison and is used to adjust the frequency of the VCO until the loop matches the phases. The output signal frequency can be a multiple of the input signal frequency. The control signal can be differential or single-ended depending on the application. Modern PLLs are typically embodied in the form of an integrated circuit as part of an electronic chip.
While various forms of PLLs have been found to be satisfactory to a certain degree, there remains room for further improvement. Indeed, VCO power supply noise can cause phase jitter, and the VCO frequency can be affected by changes in temperature. Some applications require a direct connection between the VCO and a noisy supply bump, which can make the PLL particularly exposed to noise-induced jitter. Some applications, such as clocking systems of antenna towers for instance, require maintaining clocking system operation within extreme temperature ranges, such as −40 to 125° C., for instance, which can induce VCO frequency shifts that exceed the control loop compensation capacity since the voltage range generated by the control loop is typically limited. Applications where power supply noise and extreme temperature variations are combined can cause particular design challenges for PLLs.